1. Field of the Invention
The present invention generally relates to an organic PTC (positive temperature coefficient) thermistor device and, more particularly, to the support of a thermistor element within a casing.
2. Description of the Background Art
Various PTC thermistors have long been used as protective circuit elements for protecting circuit component parts from the overcurrent. Of these various PTC thermistors, organic PTC thermistors are well known and comprise a thermistor element made from electroconductive particles such as, for example, those of carbon black or any other metal, mixed into synthetic resin of a polyolefin system such as, for example, polyethylene.
According to the background art, the thermistor element in an organic PTC thermistor is encased in a fashion shown in FIG. 9 of the accompanying drawings.
Referring to FIG. 9 for the purpose of discussion on the prior art, the organic PTC thermistor element is generally identified by 10 and is in the form of, for example, a disc having on its opposite surfaces respective, thermally-deposited metallic foils. These metallic foils serve as electrodes 11 having respective leads 12 connected thereto by means of solder deposits 13. The assembly is then encased with an outer coating 14 of synthetic resin with outer end portions of the leads 12 exposed to the outside of the outer coating 14 for electric connection with external circuit elements.
In the prior art PTC thermistor device of the construction shown in FIG. 9, it has been found that the thermistor element 10 tends to deteriorate so much as to result in a loss of stability under the influence of heat evolved during the soldering of the leads 12 to the electrodes 11 and/or the formation of the outer coating 14.
In view of the foregoing, the Japanese Laid-open Utility Model Publication No. 61-201, published in 1986, has proposed an organic PTC thermistor device free from thermal influences. According to this publication, the PTC thermistor device comprises an organic PTC thermistor element having electrodes deposited on the respective opposite surfaces thereof, which element is retained in position within a casing by means of a pair of terminal members elastically clamping the element from opposite directions while held in contact with the electrodes.
However, the PTC thermistor device disclosed in the above-mentioned publication has been found having a problem in that, when the thermistor element is heated as a result of an overcurrent induced in the element during its operation, the element, which is made from organic material as its principal component, tends to be softened to such an extent that resilient forces exerted by the terminal members and centered on the respective points of contact with the associated electrodes may cause the element to deform at two locations, corresponding respectively to the points of contact of the terminal members with the electrodes, in respective directions towards each other. In the worst case it may happen, the thickness of the thermistor element may be reduced at a portion where it is elastically clamped by the terminal members, resulting in shortcircuiting between the opposite electrodes.
The above-discussed problem may be obviated if the resilient forces applied from the terminal members to the element through the associated electrodes to retain the element in position are reduced. However, the reduction of the resilient forces may permit the element to undergo arbitrary motion within the casing under the influence of vibrations and/or impacts and also to exhibit an increased contact resistance accompanied by change in operating performance.